The present invention relates to a method of preparing a high active phase ammoxidation catalyst having improved performance and attrition resistance. In particular, the present invention is directed to a method of preparing a bismuth molybdate ammoxidation catalyst characterized by containing greater than 60% active phase and less than 40% support. The catalysts prepared by the method of the present invention exhibit improved attrition resistance while maintaining their catalytic activity to the ammoxidation of propylene or isobutylene with ammonia to produce acrylonitrile or methacrylonitrile.
Any commercial fluid bed ammoxidation catalyst comprises two essential parts. The first part is an active phase which usually is mixture of transition metal oxides promoted with alkali, alkaline earth and/or amphoteric metal oxides. This active phase is generally recognized as being responsible for the catalytic activity required for the conversion of hydrocarbons to useful products. The second part of the commercial catalyst is the inert support which is generally thought of as not participating in the catalytic reaction. The active phase of the catalyst is usually soft and becomes fine and powdery upon continued use in a reactor. The support material has oxygen atoms associated with it which are too tightly bonded to be removed under normal reactant conditions. Accordingly, the support is characterized as being hard and is added to increase the hardness (the attrition resistance) of the catalyst and make it suitable for operation in a commercial fluid bed reactor.
An appropriate blend of both the active phase and the support is crucial to obtain a suitable catalytic activity and hardness (attrition resistance) for the catalyst. Typically, the ratio of active phase to support in fluid bed catalysts is between about 50-60%:50-40%. Previously, any increase in the percent active phase increased the activity of the catalyst, but decreased the attendant attrition resistance. Conversely, any increase in the percent support increased the attrition resistance of the catalyst, but was accompanied by a decrease in the activity of the catalyst. For a clear teaching of the effects of too much active or support phase in a commercial catalyst, see U.S. Pat. No. 4,228,098 herein incorporated by reference. Accordingly, the desire to increase the percent of active material in the catalyst while maintaining its attrition resistance at a commercially acceptable level, while recognized, has until now remained elusive. The process of the present invention is directed to solving this long felt need and is particularly applicable to catalysts which have a high active phase (greater than 60% active phase) and a low support phase (less than 40% support). It is also envisioned that the method of the present invention is completely applicable to conventional commercial catalysts.